Method of producing a building panel and a building panel

ABSTRACT

A method of producing a building panel ( 1 ), including: providing a core ( 2 ), applying a balancing layer ( 6 ) having a first moisture content on a first surface ( 3 ) of the core ( 2 ), the balancing layer ( 6 ) comprising a sheet impregnated with a thermosetting binder, applying a surface layer ( 12 ) having a second moisture content on a second surface ( 4 ) of the core ( 2 ), the surface layer ( 12 ) comprising a thermosetting binder, adjusting the first moisture content of the balancing layer ( 6 ) such that the first moisture content of the balancing layer ( 6 ) is higher than the second moisture content of the surface layer ( 12 ) prior to curing, and curing the surface layer ( 12 ) and the balancing layer ( 6 ) by applying heat and pressure. Also, a semi-finished product adapted to be cured for forming a building panel ( 1 ).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 61/751,393, filed on Jan. 11, 2013. The entire contentsof U.S. Provisional Application No. 61/751,393 are hereby incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a method of forming a balancing layeron a building panel, a semi-finished product and a building panel.

TECHNICAL BACKGROUND

Traditional laminated panels, intended to be used for e.g. flooring orfurniture components, are generally produced by the following steps:

-   -   applying a melamine formaldehyde resin impregnated paper as        balancing layer on one face of a core of wood fibre based        material, for example, HDF;    -   applying a melamine formaldehyde resin impregnated printed décor        paper on the other face of the core;    -   applying a melamine formaldehyde resin impregnated transparent        overlay paper with wear resistant particles, for example,        aluminium oxide, on the décor paper; and    -   curing the resin by applying heat and pressure in a continuous        or discontinuous press to obtain a laminated product.

Typical press parameters are 40-60 bar pressure and a temperature of160-200° C. with a pressing time of 8-45 seconds. The surface layer hasgenerally a thickness of 0.1-0.2 mm, the thickness of the core variesbetween 6-12 mm and the balancing layer is about 0.1-0.2 mm thick.

This production method and product produced by such methods aregenerally referred to as the DPL process and DPL products (DirectPressure Laminate).

Recently, building panels with a wood fibre based surface have beendeveloped. Powder comprising wood fibres, a binder, preferably melamineformaldehyde resin, aluminium oxide particles and colour pigments arescattered on a core of a wood fibre based material such as HDF andpressed under heat and pressure in a continuous or discontinuous pressto obtain a product with a paper free and solid surface layer. The pressparameters are similar to DPL. A higher pressure of 40-80 bar and apressing time of 15-45 seconds may be used when the surface is formedwith deep embossing and a thickness of, for example, 0.4-0.6 mm. Thepressing temperature is generally 150-200° C. Such wood fibre basedfloors, generally referred to as Wood Fibre Floors (WFF), haveconsiderably better properties than traditional laminate floors since athicker and more impact-resistant and wear-resistant surface with deepembossing may be produced in a cost efficient way.

Both DPL products and panels having a wood fibre based surface have abalancing layer arranged on the rear face of the core. The balancinglayer may be a balancing paper or powder-based balancing layercomprising wood fibres and a binder. The core with the upper and lowerlayers is moved into a press and pressed under heat and pressure suchthat the layers are cured and attached to the core.

The layers on the front face and the rear face of the core are exposedto a first shrinking when the thermosetting resin in the upper and lowerlayer cures during pressing. The balancing layer at the rear facebalances the tension that is created by the surface layer of the frontface and the panel is substantially flat with a small convex backwardbending when it leaves the press. Such first shrinking and balancing ofthe panel is below referred to as “pressing balancing”. The secondtemperature shrinking, when the panels is cooled from about 150-200° C.to room temperature, is also balanced by the balancing layer and thepanel is essentially flat. The second balancing is below referred to as“cooling balancing”. A small convex backward bending is preferred sincethis counteracts upward bending of the edges in dry conditions when therelative humidity may go down to 20% or lower during wintertime.

A problem is that this essentially flat panel comprises tension forcescaused by the shrinking of the surface and balancing layers duringpressing and during cooling to room temperature.

The surface layer and the core will swell in summertime when the indoorhumidity is high and shrink in wintertime when the indoor humidity islow. The panels will shrink and expand and a cupping of the edges maytake place. The balancing layer is used to counteract such cupping. Inthe installed floor, the balancing layer is used to work as a diffusionbarrier for moisture from the underlying floor, and to minimize theimpact of the surrounding climate. Consequently, the balancing layer isadapted balance shrinking and expansion caused by both pressing, coolingand climate changes.

It is known that a balancing layer comprising a powder mix of woodfibres and a thermosetting binder can be used to balance the surfacelayer. A method for producing such a balancing layer is described in WO2012/141647.

US 2010/0239820 describes a method of producing a laminated decorativeplate wherein an unimpregnated decorative layer is applied onto a layerof glue arranged on the core. A counteracting layer is arranged on theother side of the core, and it is described that neutral paper withsolid or liquid resin as a glue is suitable for the counteracting layer.The counteracting layer is adapted to prevent the finished plate fromwarping or bending as a result of influences of temperature andhumidity. WO 2010/084466 discloses a balancing layer substantiallyconsisting of a synthetic material layer, which is free from materialsheets. The synthetic material is provided in liquid form.

SUMMARY

It is an object of at least certain embodiments of the present inventionto provide an improvement or alternative over the above describedtechniques and prior art.

A further object of at least certain embodiments of the presentinvention is to provide a method reducing the cost for providing abuilding panel with a balancing layer.

A further object of at least certain embodiments of the presentinvention is to reduce the amount of thermosetting binder in thebalancing layer.

At least some of these and other objects and advantages that will beapparent from the description have been achieved by a method ofproducing a building panel. The method comprises:

providing a core,

applying a balancing layer on a first surface of the core, wherein thebalancing layer has a first moisture content and comprises a sheetimpregnated with a thermosetting binder,

applying a surface layer on a second surface of the core, wherein thesurface layer has a second moisture content and comprises athermosetting binder,

adjusting the first moisture content of the balancing layer such thatthe first moisture content of the balancing layer is higher than thesecond moisture content of the surface layer prior to curing by applyingheat and pressure, and

curing the surface layer and the balancing layer by applying heat andpressure.

By moisture content is meant water present in any form.

The balancing layer is adapted to counteract shrinking forces formed bythe surface layer during pressing and cooling such that the buildingpanel remains essentially flat in room temperature after pressing. Theshrinking forces formed by the thermosetting binder of the surface layerduring pressing and curing are balanced or counteracted by the shrinkingforced formed by the thermosetting binder of the balancing layer duringpressing. By arranging a balancing layer on the core opposite thesurface layer, the shrinking forces formed by the thermosetting binderin the surface layer and by the thermosetting binder in the balancinglayer are balanced against each other.

An advantage of at least embodiments of the invention is that byadjusting the moisture content of the balancing layer to be higher thanthe moisture content of the surface layer prior to curing, for example,by applying water on the core and/or on the balancing layer, thebalancing layer can balance larger shrinking forces formed by thesurface layer during pressing and cooling. By increasing the moisturecontent of the balancing layer prior to curing, the amount ofthermosetting binder of the balancing layer can be reduced. A thinnerpaper, i.e. a paper having reduced weight per square meter, can also beused. The reduced amount of the thermosetting binder of the balancinglayer is compensated by the effects of the higher moisture content ofthe balancing layer.

Pressing tests show that a suitable moisture content in a balancinglayer comprising a thermosetting binder such as melamine formaldehydemay increase the shrinking forces during curing and cooling and thatthis may be used to reduce the amount of the thermosetting binder in thebalancing layer. It is preferred that the moisture content in thethermosetting binder of the balancing layer prior to pressing is higherthan the general moisture content of about 4.5-6% that is the case whenconventional impregnated paper is used as a balancing layer. Themoisture content of the balancing layer should preferably exceed themoisture content of the surface layer. The moisture content of thebalancing layer preferably exceeds 10% of the total weight of thebalancing layer. In some application it may be an advantage to use evenhigher moisture contents, for example, exceeding 20%. The moisturecontent is measured prior to pressing. The moisture content in aconventional backing paper is usually limited by the fact that thepapers are stacked on a pallet and too high moisture content causessticking and single paper sheets cannot be taken from the pallet.

The moisture in the balancing layer contributes to the increasedbalancing capacity of the balancing layer in several ways. The moisturein the balancing layer facilitates and improves heat transfer from theheated press plates into the balancing layer, thus accelerating and/orincreasing cross-linking of the thermosetting resin. The moisturecontent also influences curing of the thermosetting resin by increasingor facilitating floating of the thermosetting binder, which may resultin a higher degree of cross-linking. The moisture content of thebalancing layer may also influence the core of the building panel. Themoisture content of the balancing layer may make the core, for example,a core of a wood-based material, more formable during pressing. A smallconvex backward bending of the core may be achieved, counteracting anupward bending of the edges of the surface layer. The method maycomprise adjusting the moisture content of the balancing layer and/orthe surface layer such that the moisture content of the balancing layeris higher than the moisture content of the surface layer.

The step of adjusting the first moisture content of the balancing layermay comprise applying water or steam to the first surface of the coreprior to applying the balancing layer.

The step of adjusting the first moisture content of the balancing layercomprises applying water or steam to the balancing layer. Adjusting thefirst moisture content of the balancing layer may also be performed byapplying water or steam to both the first surface of the core and to thebalancing layer prior to curing by applying heat and pressure.

The first moisture content of the balancing layer may be about 6-30%,preferably 8-20% of the total weight of the balancing layer prior tocuring.

The first moisture content of the balancing layer may exceed 10%,preferably exceeds 20%, more preferably exceeds 30%, of the total weightof the balancing layer.

The thermosetting binder may be of the same type in the surface layerand in the balancing layer.

The thermosetting binder of the surface layer and/or the balancing layermay be melamine formaldehyde resin. The thermosetting binder of thesurface binder and/or the balancing layer may also be urea melamineformaldehyde or a combination of urea/melamine formaldehyde. Any otheramino resin is also possible, such as phenol formaldehyde resin.

The sheet impregnated with the thermosetting binder may be a paper sheetimpregnated with the thermosetting binder. The paper sheet is preferablya melamine formaldehyde impregnated paper.

The surface layer may comprise a décor paper, preferably a resinimpregnated décor paper, more preferably a melamine formaldehydeimpregnated décor paper. The surface layer may also comprise an overlaypaper, preferably a resin impregnated overlay paper comprising wearresistant particles.

The surface layer may comprise a layer comprising a thermosetting binderand at least one pigment.

The amount of the thermosetting binder in the balancing layer may belower than the amount of the thermosetting binder in the surface layer.

The core may be wood-based material such as a MDF, HDF, plywood, OSB,Wood Plastic Composite (WPC), etc.

According to a second aspect of the invention, a semi-finished productfor forming a building panel after curing is provided. The semi-finishedproduct comprises a core having a first surface and a second surfaceopposite the first surface, a balancing layer arranged on the firstsurface of the core, wherein the balancing layer comprises a sheetimpregnated with a thermosetting binder, a surface layer arranged on thesecond surface of the core, wherein the surface layer comprises athermosetting binder, wherein a first moisture content of the balancinglayer is higher than a second moisture content of the surface layerprior to curing.

By curing is meant curing by applying heat and pressure, i.e. reactingthe thermosetting binder to C-stage.

The second aspect incorporates all the advantages of the fifth aspect ofthe invention, which previously have been discussed in relation to thefirst aspect, whereby the previous discussion is applicable also for thebuilding panel.

The thermosetting binder of the balancing layer may B-stage.

The balancing layer is adapted to counteract forces formed by thesurface layer during pressing and cooling such that the building panelremains essentially flat in room temperature after pressing and curing.

The first moisture content of the balancing layer may be about 6-30%,preferably 8-20%, of the total weight of the balancing layer prior tocuring.

The first moisture content of the balancing layer may exceed 10%,preferably exceeds 20%, more preferably exceeds 30%, of the total weightof the balancing layer prior to curing.

The thermosetting binder may be of the same type in the surface layerand in the balancing layer.

The thermosetting binder of the surface layer and/or the balancing layermay be melamine formaldehyde resin. The thermosetting binder of thesurface binder and/or the balancing layer may also be urea melamineformaldehyde or a combination of urea/melamine formaldehyde.

The sheet impregnated with the thermosetting binder may be a paperimpregnated with the thermosetting binder, preferably a melamineformaldehyde impregnated paper.

The surface layer may comprises a décor paper, preferably a resinimpregnated décor paper, more preferably a melamine formaldehydeimpregnated décor paper. The surface layer may also comprise an overlaypaper, preferably a resin impregnated paper comprising wear resistantparticles.

The surface layer may comprise a layer comprising a thermosetting binderand at least one pigment.

According to third aspect of the invention, a method of producing abuilding panel is provided. The method comprises providing a core,

applying a balancing layer on a first surface of the core, wherein thebalancing layer has a first moisture content and comprises a B-stagethermosetting binder, applying a surface layer on a second surface ofthe core, wherein the surface layer has a second moisture content andcomprises a B-stage thermosetting binder, adjusting the first moisturecontent of the balancing layer such that the first moisture content ofthe balancing layer is higher than the second moisture content of thesurface layer prior to curing, and

curing the thermosetting binder of the surface layer and the balancinglayer to C-stage by applying heat and pressure.

Thermosetting binders may be classified as A-, B-, and C-stage accordingto their extent of reaction compared to the extent of reaction atgelation. In an A-stage thermosetting binder, the extent of reaction isless than the extent of reaction at gelation, i.e. uncured. A B-stagethermosetting binder is close to the gel point, i.e. semi-cured. AC-stage thermosetting binder is well past the gel point, i.e. cured. TheA-stage thermosetting binder is soluble and fusible. The B-stagethermosetting resin is still fusible but is barely soluble. The C-stagethermosetting binder is highly crosslinked and both infusible insoluble.(Principles of Polymerization, George Odian, 3^(rd) edition).

The B-stage thermosetting binder applied for the balancing layer may bereacted to the B-stage in a previous step, such as reacted to theB-stage in a dried thermosetting resin impregnated paper, or as aspray-dried thermosetting resin. By a B-stage thermosetting binder isnot meant a thermosetting binder in liquid form. In the B-stage,reactivity of the thermosetting binder remains. In the C-stage, thethermosetting binder has no remaining reactivity, or at least almost noreactivity. Thermosetting binders in the B-stage may have a varyingdegree of crosslinking, from a low degree of crosslinking being close toA-stage, to a high degree of crosslinking being close to C-stage.

The B stage thermosetting binder of the balancing layer may be presentin form of, for example, a sheet impregnated with the thermosettingbinder such as a paper sheet impregnated with the thermosetting binder,a thermosetting binder applied in powder form.

The balancing layer is adapted to counteract shrinking forces formed bythe surface layer during pressing and cooling such that the buildingpanel remains essentially flat in room temperature after pressing. Theshrinking forces formed by the thermosetting binder of the surface layerduring pressing and curing are balanced or counteracted by the shrinkingforced formed by the thermosetting binder of the balancing layer duringpressing.

By arranging a balancing layer on the core opposite the surface layer,the shrinking forces formed by the thermosetting binder in the surfacelayer and by the thermosetting binder in the balancing layer arebalanced against each other.

An advantage of at least embodiments of the invention is that byadjusting the moisture content of the balancing layer to be higher thanthe moisture content of the surface layer prior to curing, for example,by applying water on the core and/or on the balancing layer, thebalancing layer can balance larger shrinking forces formed by thesurface layer during pressing and cooling. By increasing the moisturecontent of the balancing layer prior to curing, the amount ofthermosetting binder of the balancing layer can be reduced. A thinnerpaper, i.e. a paper having reduced weight per square meter, can also beused. The reduced amount of the thermosetting binder of the balancinglayer is compensated by the effects of the higher moisture content ofthe balancing layer.

Pressing tests show that a suitable moisture content in a balancinglayer comprising a thermosetting binder such as melamine formaldehydemay increase the shrinking forces during curing and cooling and thatthis may be used to reduce the amount of the thermosetting binder in thebalancing layer. It is preferred that the moisture content in thethermosetting binder of the balancing layer prior to pressing is higherthan the general moisture content of about 6% that is the case when aconventional impregnated paper is used as a balancing layer. Themoisture content of the balancing layer should preferably exceed themoisture content of the surface layer. The moisture content of thebalancing layer preferably exceeds 10% of the total weight of thebalancing layer. In some application it may be an advantage to use evenhigher moisture contents, for example, exceeding 20%. The moisturecontent is measured prior to pressing. The moisture content in aconventional backing paper is usually limited by the fact that thepapers are stacked on a pallet and too high moisture content causessticking and single paper sheets cannot be taken from the pallet.

The moisture in the balancing layer contributes to the increasedbalancing capacity of the balancing layer in several ways. The moisturein the balancing layer facilitates and improves heat transfer from theheated press plates into the balancing layer, thus accelerating and/orincreasing cross-linking of the thermosetting resin. The moisturecontent also influences curing of the thermosetting resin by increasingor facilitating floating of the thermosetting binder, which may resultin a higher degree of cross-linking. The moisture content of thebalancing layer may also influence the core of the building panel. Themoisture content of the balancing layer may make the core, for example,a core of a wood-based material, more formable during pressing. A smallconvex backward bending of the core may be achieved, counteracting anupward bending of the edges of the surface layer.

The step of adjusting the first moisture content of the balancing layermay comprise applying a water or steam to the first surface of the coreprior to applying the balancing layer.

The step of adjusting the first moisture content of the balancing layermay comprise applying a water or steam to the balancing layer.

The first moisture content of the balancing layer may be about 6-30%,preferably 8-20% of the total weight of the balancing layer prior tocuring.

The first moisture content of the balancing layer may exceed 10%,preferably exceeds 20%, more preferably exceeds 30%, of the total weightof the balancing layer. The thermosetting binder may be of the same typein the surface layer and in the balancing layer.

The thermosetting binder of the surface layer and/or the balancing layermay be melamine formaldehyde resin. The thermosetting binder of thesurface binder and/or the balancing layer may also be urea melamineformaldehyde or a combination of urea/melamine formaldehyde. Any otheramino resin is also possible, such as phenol formaldehyde resin.

The balancing layer may comprise a sheet impregnated with thethermosetting binder, preferably a paper sheet impregnated withthermosetting binder.

The step of applying the balancing layer may comprise applying thethermosetting binder in powder form.

The balancing layer may comprise at least 80 wt % of the thermosettingbinder, preferably at least 90 wt % of the thermosetting binder.

The amount of the thermosetting binder in the balancing layer may belower than the amount of the thermosetting binder in the surface layer.The surface layer may comprises a décor paper, preferably a resinimpregnated décor paper, more preferably a melamine formaldehydeimpregnated décor paper. The surface layer may also comprise an overlaypaper, preferably a resin impregnated paper comprising wear resistantparticles.

The surface layer may comprise a layer comprising a thermosetting binderand at least one pigment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will by way of example be described in more detailwith reference to the appended schematic drawings, which showembodiments of the present invention.

FIG. 1 a-e illustrates a method of producing a building panel accordingto an embodiment of the present invention.

FIG. 2 a-e illustrates a method of producing a building panel accordingto an embodiment of the present invention.

FIG. 3 illustrates a floor panel provided with a mechanical lockingsystem.

FIG. 4 a-e illustrates a method of producing a building panel accordingto an embodiment.

DETAILED DESCRIPTION

FIG. 1 a-e illustrates a method of producing a building panel 1according to an embodiment of the present invention. The building panel1 may be a floor panel, a wall panel, a ceiling panel, a furniturecomponent, etc.

The method comprises providing a core 2. The core 2 is preferably awood-based board such as MDF or HDF. The core 2 may also be a particleboard, OSB or plywood. The core may also be a Wood Plastic Composite(WPC). The core 2 comprises a first surface 3 and a second surface 4opposite the first surface 3. The core 2 may be arranged on a conveyorbelt 5 conveying the core 2 between the steps described below withreference to FIG. 1 a-e.

Steam or water is applied, preferably by spraying, on the first surface3 of the core 2 by a steam or water application device 17 as shown inFIG. 1 a. 10-45 g/m2, preferably 15-30 g/m2, of water may be applied.The water or steam may be applied as an aqueous solution. The aqueoussolution may comprise release and wetting agents and catalysts.

In FIG. 1 b, a balancing layer 6 is applied to the first surface 3 ofthe core 2. In the embodiment shown in FIG. 1 a-e, the balancing layercomprises a sheet impregnated with a thermosetting binder, such as apaper sheet impregnated with a thermosetting binder. As an alternativeto a paper sheet, a glass fibre sheet or non-woven may be provided. Thethermosetting binder may be an amino resin such as melamineformaldehyde, phenol formaldehyde, urea formaldehyde, or a combinationthereof. The thermosetting binder may be B-stage when applied as thebalancing layer 6, i.e. close to the gel point.

Alternatively, or as a complement to applying steam or water on thefirst surface 3 of the core 2, steam or water may be applied on thebalancing layer 6 when arranged on the core 2, as shown in FIG. 1 b.

By applying steam or water on the core 2 and/or on the balancing layer6, moisture is added to the balancing layer 6 and/or a portion of thecore 2 adjacent the balancing layer 6 such that the moisture content ofthe balancing layer is adjusted. Pressing tests show that a suitablemoisture content in the balancing layer 6 comprising a thermosettingbinder such as melamine formaldehyde may increase the shrinking forcesduring curing and cooling and that this may be used to reduce thecontent of the thermosetting binder in the balancing layer 6. It ispreferred that the moisture content in the thermosetting binder of thebalancing layer 6 prior to pressing is higher than the general moisturecontent of about 4.5-6% that is the case when conventional impregnatedpaper is used as a balancing layer. The moisture content of thebalancing layer 6 should preferably exceed the moisture content of thesurface layer 12. The moisture content of the balancing layer 6 may be6-30%, preferably 8-20%, of the total weight of the balancing layer 6.In some application it may be an advantage to use even higher moisturecontents, for example, exceeding 20%. The moisture content is measuredprior to pressing.

In order to further adjust the moisture content of the balancing layer 6prior to pressing, the balancing layer 6 may be heated by, for example,IR or hot air to dry the balancing layer 6. The thermosetting binderremains essentially in B-stage.

The core 2 with the balancing layer 6 may be handled. For example, thecore may be turned such that the balancing layer 6 faces the conveyorbelt 5 as shown in FIG. 1 c. The core 2 with the balancing layer 6 maybe stacked on a pallet for intermediate storage, or may be stored andtransported to another plant. Thereby, the core 2 having the balancinglayer 6 attached thereto may be produced in a different process than thesubsequent application of the surface layer, and at a differentlocation. A surface layer 12 is applied to the second surface 4 of thecore 2 as shown in FIG. 1 d. The surface layer 12 may be a décor paper14 such as a resin impregnated paper. The resin impregnated paper 14 ispreferably a melamine or urea formaldehyde impregnated paper. Thesurface layer 12 may further comprise an overlay paper 14 as shown inFIG. 1 d. The surface layer 12 may be a DPL. The décor paper 13 isarranged directly on the second surface 4 of the core 2. The overlaypaper 14 is arranged on the décor paper 13. The décor paper 13 ispreferably impregnated with a resin, preferably impregnated withmelamine formaldehyde resin. The décor paper 13 comprises preferably adecorative print. The overlay paper 14 is preferably also impregnatedwith a resin, for example, melamine formaldehyde resin. The overlaypaper 14 comprises preferably wear resistant particles such as aluminiumoxide. The overlay paper 14 is preferably transparent.

Alternatively, the surface layer 12 may be a wood fibre based powdercomprising wood fibres, a thermosetting resin and wear resistantparticles. It is also contemplated that the surface layer 12 may be ofany other type, for example, a veneer layer, or a combination of a décorpaper and a wood fibre based surface. The surface layer 12 may also be alayer of a thermosetting binder such as melamine formaldehyde or ureaformaldehyde. The layer may consist essentially of the thermosettingbinder but may also comprise decorative elements such as pigments andwear resistant particles such as aluminium oxide. The surface layer 12comprises in this embodiment no paper.

In a preferred embodiment, the binder of the balancing layer 6 and thebinder of the surface layer 12 is of the same type. Preferably, melamineformaldehyde resin is used both in the surface layer 12 and in thebalancing layer 6.

A semi-finished product as shown in FIG. 1 d is thereby obtained. Thesemi-finished product comprises the core 2 having the balancing layer 6and the surface layer 12 arranged thereon. The balancing layer 6comprises a sheet impregnated with the thermosetting binder. Themoisture content of the balancing layer 6 is higher than the moisturecontent of the surface layer 12 as measured prior to curing by applyingheat and pressure.

The moisture content of the balancing layer 6 prior to curing may beabout 6-30%, preferably 8-20%, of the total weight of the balancinglayer prior to curing. The balancing layer 6 and the surface layer 12 isthereafter cured by applying heat and pressure in a press. Thethermosetting binder of the balancing layer 6 and the surface layer 12is cured to its C stage. By curing and pressing the surface layer 12 andthe balancing layer 6 to the core 2, the surface layer 12 and thebalancing layer 6 are adhered to the core 2. Thereby, a building panel 1comprising the core 2, the surface layer 12 and the balancing layer 6 isobtained, which is shown in FIG. 1 e.

After pressing, the balancing layer 6 balances the tension created bythe surface layer 12 during curing such that the building panel 1remains essentially flat after pressing and cooling (“pressingbalancing” and “cooling balancing”). By essentially flat is meant acupping less than 2 mm/m. A small convex backward bending being lessthan 2 mm/m is preferred. The thermosetting binder of the balancinglayer 6 balances forces created by the thermosetting binder of thesurface layer 12. By applying water on the core 2 and/or on thebalancing layer 6, the balancing layer 6 may balance larger shrinkingforces formed by the surface layer 12 during pressing and cooling.Thereby, the amount of the thermosetting binder of the balancing layer 6can be reduced. As an example, the amount of the thermosetting binder inthe balancing layer 6 may about 75% of the amount of the thermosettingbinder in the surface layer

FIGS. 2 a-e illustrates a method of producing a building panel 1according to an embodiment of the present invention. The building panel1 may be a floor panel, a wall panel, a ceiling panel, a furniturecomponent, etc.

The method comprises providing a core 2. The core 2 is preferably awood-based board such as MDF or HDF. The core 2 may also be a particleboard, OSB or plywood. The core may also be a Wood Plastic Composite(WPC). The core 2 comprises a first surface 3 and a second surface 4opposite the first surface 3. The core 2 may be arranged on a conveyorbelt 5 conveying the core 2 between the steps described below withreference to FIG. 2 a-e.

Steam or water is applied, preferably by spraying, on the first surface3 of the core 2 by a steam or water application device 17 as shown inFIG. 2 a. 10-45 g/m2, preferably 15-30 g/m2, of water may be applied.The water or steam may be applied as an aqueous solution. The aqueoussolution may comprise release and wetting agents and catalysts.

In FIG. 2 b, a balancing layer 6 is applied to the first surface 3 ofthe core 2. The balancing layer comprises a B-stage thermosettingbinder. The thermosetting binder may be an amino resin such as melamineformaldehyde, phenol formaldehyde, urea formaldehyde, or a combinationthereof. The thermosetting binder may have reacted to the B-stage in aprevious step, such as in a dried resin impregnated paper, or as inspray-dried thermosetting binder. The balancing layer 6 comprising theB-stage thermosetting binder may be applied as a sheet impregnated withthe thermosetting binder, for example, a paper sheet, as described abovewith reference to FIG. 1 a-e. The B-stage thermosetting binder may alsobe applied in powder form for forming the balancing layer 6, as shown inFIGS. 2 a-e.

In FIG. 2 b, a B-stage thermosetting binder is applied in powder form onthe first surface 3 of the core 2 for forming a balancing layer 6. Thethermosetting binder is preferably melamine formaldehyde resin in drypowder form, such as spray dried melamine formaldehyde resin. Additivessuch as wetting agents, release agents, catalyst, may be added to thethermosetting binder. The thermosetting powder 7 preferably has anaverage particle size of in the range of about 50-150 microns.

The thermosetting powder 7 is preferably scattered on the first surface3 of the core 2 by a scattering unit 8. The thermosetting binder inpowder form is applied on the first surface 3 of the core 2 such as alayer forming a balancing layer 6 is formed. The scattered layercomprises preferably 50-150 g/m2, such as 50-100 g/m2, of spray driedthermosetting binder such as spray dried melamine formaldehydeparticles. The scattered layer preferably corresponds to a thickness ofabout 0.1-0.5 mm of powder or about 0.1-0.2 mm of a pressed and curedthermosetting binder layer such as a melamine formaldehyde layer.

Since water or steam has been applied to the first side 3 of the core 2prior to applying the thermosetting binder, the thermosetting bindergets tacky and sticks together such that a layer of the thermosettingbinder is formed and such that the layer of the thermosetting binderadheres to the core. Thereby, it is possible to handle the core withoutthe thermosetting binder falling off the core.

Alternatively, or as a complement to applying steam or water on thefirst surface 3 of the core 2, steam or water may be applied on thebalancing layer 6 when arranged on the core 2, as shown in FIG. 2 b.

By applying steam or water on the core 2 and/or on the balancing layer6, moisture is added to the balancing layer 6 and/or a portion of thecore 2 adjacent the balancing layer 6 such that the moisture content ofthe balancing layer is adjusted. Pressing tests show that a suitablemoisture content in the balancing layer 6 comprising a thermosettingbinder such as melamine formaldehyde may increase the shrinking forcesduring curing and cooling and that this may be used to reduce thecontent of the thermosetting binder in the balancing layer 6. It ispreferred that the moisture content in the thermosetting binder of thebalancing layer 6 prior to pressing is higher than the general moisturecontent of about 4.5-6% that is the case when conventional impregnatedpaper is used as a balancing layer. The moisture content of thebalancing layer 6 should preferably exceed the moisture content of thesurface layer 12. The moisture content of the balancing layer 6 may be6-30%, more preferably 8-20%, of the total weight of the balancing layer6. In some application it may be an advantage to use even highermoisture contents, for example, exceeding 20%. The moisture content ismeasured prior to pressing. In order to further adjust the moisturecontent of the balancing layer 6 prior to pressing, the balancing layer6 may be heated by, for example, IR or hot air to dry the balancinglayer 6. The balancing layer 6 remains essentially in B-stage, or atleast not completely in C-stage.

The core 2 with the balancing layer 6 may be handled, for example,turned such that the balancing layer 6 faces the conveyor belt 5 asshown in FIG. 2 c. The core 2 with the balancing layer 6 may be stackedon a pallet for intermediate storage, or may be stored and transportedto another plant. Thereby, the core 2 having the balancing layer 6attached thereto may be produced in a different process than thesubsequent application of the surface layer, and at a differentlocation.

When the semi-finished product has been turned, a surface layer 12 isapplied to the second surface 4 of the core 2 as shown in FIG. 2 d. Thesurface layer 12 may be a décor paper 14 such as a resin impregnatedpaper. The resin impregnated paper 14 is preferably a melamine or ureaformaldehyde impregnated paper. In the embodiment shown in FIG. 2 d, thesurface layer 12 further comprises an overlay paper 14. The surfacelayer 12 may be a DPL. The décor paper 13 is arranged directly on thesecond surface 4 of the core 2. The overlay paper 14 is arranged on thedécor paper 13. The décor paper 13 is preferably impregnated with aresin, preferably impregnated with melamine formaldehyde resin. Thedécor paper 13 comprises preferably a decorative print. The overlaypaper 14 is preferably also impregnated with a resin, for example,melamine formaldehyde resin. The overlay paper 14 comprises preferablywear resistant particles such as aluminium oxide. The overlay paper 14is preferably transparent.

Alternatively, the surface layer 12 may be a wood fibre based powdercomprising wood fibres, a thermosetting resin and wear resistantparticles. It is also contemplated that the surface layer 12 may be ofany other type, for example, a veneer layer, or a combination of a décorpaper and a wood fibre based surface.

The surface layer 12 may also be a layer of a thermosetting binder suchas melamine formaldehyde or urea formaldehyde. The layer may consistessentially of the thermosetting binder. A layer consisting essentiallyof the thermosetting binder may also include decorative elements, suchas pigments, and wear resistant particles, such as aluminium oxide. Thesurface layer 12 comprises, in this embodiment, no paper.

In a preferred embodiment, the binder of the balancing layer 6 and thebinder of the surface layer 12 is of the same type. Preferably, melamineformaldehyde resin is used both in the surface layer 12 and in thebalancing layer 6.

A semi-finished product as shown in FIG. 2 d is thereby obtained. Thesemi-finished product comprises the core 2 having the balancing layer 6and the surface layer 12 arranged thereon. The balancing layer 6comprises a sheet impregnated with the thermosetting binder. Themoisture content of the balancing layer 6 is higher than the moisturecontent of the surface layer 12 as measured prior to curing by applyingheat and pressure.

The moisture content of the balancing layer 6 prior to curing preferablymay be 6-30%, more preferably 8-20%, of the total weight of thebalancing layer 6.

The balancing layer 6 and the surface layer 12 is thereafter cured byapplying heat and pressure in a press. The thermosetting binder of thebalancing layer 6 and the surface layer 12 is cured to its C stage. Bycuring and pressing the surface layer 12 and the balancing layer 6 tothe core 2, the surface layer 12 and the balancing layer 6 are adheredto the core 2. Thereby, a building panel 1 comprising the core 2, thesurface layer 12 and the balancing layer 6 is obtained, which is shownin FIG. 2 e.

After pressing, the balancing layer 6 balances the tension created bythe surface layer 12 during curing such that the building panel 1remains essentially flat after pressing and cooling (“pressingbalancing” and “cooling balancing”). By essentially flat is meant acupping less than 2 mm/m. A small convex backward bending being lessthan 2 mm/m is preferred. The thermosetting binder of the balancinglayer 6 balances forces created by the thermosetting binder of thesurface layer 12. By applying water on the core 2 and/or on thebalancing layer 6, the balancing layer 6 may balance larger shrinkingforces formed by the surface layer 12 during pressing and cooling.Thereby, the amount of the thermosetting binder of the balancing layer 6can be reduced.

The balancing layer 6 described above with reference to FIG. 2 a-ecomprises at least 80% by weight of the thermosetting binder, preferablyat least 90% by weight of the thermosetting binder. In addition to thethermosetting binder, the balancing layer 6 may comprise variousadditives. The balancing layer 6 is in this embodiment paper free, i.e.comprises no paper. In a preferred embodiment, the balancing layer 6comprises only the thermosetting binder and optional additives. Thebalancing layer may consist of, or essentially consist of, thethermosetting binder. It is also contemplated that filler particles maybe applied to the balancing layer 6, preferably in an amount less than20% by weight of the balancing layer 6, more preferably in an amountless than 10% by weight of the balancing layer 6. Filler particles mayinclude wood fibres, aluminium oxide sand or other minerals, etc. Thefiller particles may be scattered onto the layer of the thermosettingbinder. The filler particles may be used to reinforce the balancinglayer 6 or to facilitate scattering. The filler particles may also beused to increase the tension forces that may be formed by the balancinglayer 6.

It is also contemplated that the water or steam may be applied to thebalancing layer 6 just before pressing. The semi-finished productcomprising the core 2 and the balancing layer 6 as described above withreference to FIG. 1 a-e and 2 a-e may be stored and transported. Priorto pressing, water or steam is applied to the balancing layer 6 suchthat the moisture content of the balancing layer 6 is higher than themoisture content of the surface layer being applied to the second side 4of the core 2. The surface layer 12 and the balancing layer 6 isthereafter cured and attached to the core 2 by applying heat andpressure. A building panel 1 is thereby formed, as shown in FIGS. 1 eand 2 e.

The building panel 1 as described above may be a floor panel, a wallpanel, a ceiling panel, a furniture component, etc. In the embodimentwherein the building panel is a floor panel, the floor panel 1′ may beprovided with a mechanical locking system as shown in FIG. 3. The floorpanel 1′ shown in FIG. 3 is provided with a mechanical locking systemfor locking the floor panel 1′ to adjacent floor panels horizontallyand/or vertically. The mechanical locking system comprises at a firstedge of the floor panel 1′ a tongue groove 26 adapted to receive atongue 25 of an adjacent floor panel, and a locking strip 22 providedwith a locking element 23 adapted to cooperate with a locking groove 24of an adjacent floor panel and lock the floor panel 1′ in a horizontaldirection to the adjacent floor panel. The mechanical locking systemfurther comprises at a second edge a locking groove 24 adapted toreceive a locking element 23 of an adjacent floor panel, and a tongue 25adapted cooperate with a tongue groove 26 of an adjacent floor panel andlock the panel 1′ in a vertical direction. The mechanical locking systemis formed in the core 2 of the floor panel 1′. Both long side edges andshort side edges of the floor panel 1′ may be provided with a mechanicallocking system. Alternatively, long side edges of the floor panel 1′ maybe provided with the mechanical locking system for horizontally andvertically locking, and the short side edges may be provided with amechanical locking system for horizontally. The mechanical lockingsystem may be of the type disclosed in WO 2007/015669. The binder of thebalancing layer will during pressing penetrate into the core materialand reinforce the outer parts of the first surface 3 of the core 2 wherethe locking strip 22 is formed. The high binder content of the balancinglayer 6 will increase the locking strengths of the mechanical lockingsystem.

Another embodiment of a method of producing a building panel 1 will nowbe described with reference to FIG. 4 a-e. The method comprisesproviding a core 2. The core 2 is preferably a wood-based board such asMDF or HDF. The core 2 may also be an OSB or plywood. The core 2comprises a first surface 3 and a second surface 4 opposite the firstsurface 3. The core 2 may be arranged on a conveyor belt 5 conveying thecore 2 between the steps described below with reference to FIG. 4 a-e.

FIG. 4 a illustrates that a thermosetting binder is applied on the firstsurface 3 of the core 3 for forming a balancing layer 6. Thethermosetting binder is applied as a liquid. The thermosetting binder isdissolved in a solvent, preferably a water based solvent, thus forming aliquid. The thermosetting binder is preferably melamine formaldehyderesin. Additives such as wetting and release agents, catalyst may beadded to the solvent.

A layer of the liquid thermosetting binder 7′ is applied to the firstsurface 3 of the core 2 for forming the balancing layer 6. Thereafter,the layer of the thermosetting binder is preferably dried as shown inFIG. 4 b. A heating apparatus 16, preferably an IR heating apparatus orhot air, is provided for drying the layer of the thermosetting binder.

Preferably, several layers of the thermosetting binder are applied tothe first surface 3 of the core 2 for forming the balancing layer 6.Preferably, a drying step is provided between application of each layer.

The layer or layers of the thermosetting binder is adapted to form abalancing layer 6 of the building panel 1.

Drying of the layer or layers of the thermosetting binder is performedat a temperature wherein the thermosetting binder remains essentially inB-stage. The object of the heating step is to obtain a layer which istouch dry such that the core with the thermosetting layer can behandled.

The core 2 having the thermosetting binder arranged thereon for formingthe balancing layer 6 forms a semi-finished product, which is shown inFIG. 4 c. The semi-finished product comprises the core 2 and the layerof the thermosetting binder for forming the balancing layer 6. In thesemi-finished product, the thermosetting binder remains essentially inB-stage.

The semi-finished product may be handled as a separate product due tothe balancing layer being dried and adhered to the core 2. For example,the semi-finished may be stacked on a pallet for intermediate storage,or may be stored and transported to another plant. Thereby, the core 2having the layer of thermosetting binder for forming the balancing layer6 attached thereto may be produced in a different process than thesubsequent application of the surface layer 12, and at a differentlocation.

The semi-finished product may also be turned 180° such that thebalancing layer 6 of the thermosetting binder is directed downwards, forexample, towards the conveyor belt 5 as shown in FIG. 4 c.

When the semi-finished product has been turned, a surface layer 12 maybe applied on the second surface 4 of the core 2 as shown in FIG. 4 d.In the shown embodiment, the surface layer 12 comprises a décor paper 13and an overlay paper 14. The surface layer 12 may be a DPL. The décorpaper 13 is arranged directly on the second surface 4 of the core 2. Theoverlay paper 14 is arranged on the décor paper 13. The décor paper 13is preferably impregnated with a resin, preferably impregnated withmelamine formaldehyde resin. The décor paper 13 comprises preferably adecorative print. The overlay paper 14 is preferably also impregnatedwith a resin, for example, melamine formaldehyde resin. The overlaypaper 13 comprises preferably wear resistant particles such as aluminiumoxide. The overlay paper 13 is preferably transparent.

In a preferred embodiment, the binder of the balancing layer 6 and thebinder of the surface layer 12 is of the same type. Preferably, melamineformaldehyde resin is used both in the surface layer 12 and in thebalancing layer 6.

Alternatively, the surface layer 12 may be a wood fibre based powdercomprising wood fibres, a thermosetting binder, preferably melamineformaldehyde resin, and wear resistant particles, such as aluminiumoxide. It is also contemplated that the surface layer may be of anyother type, for example, a veneer layer, or a combination of a décorpaper and a wood fibre based surface or a combination of a veneer layerand a wood fiber based surface.

The thermosetting binder of the layer for forming the balancing layer 6and of the surface layer 12 is thereafter cured by applying heat andpressure in a press. Thereby, the balancing layer 6 is formed. By curingand pressing the surface layer 12 and the balancing layer 6 to the core2, the surface layer 12 and the balancing layer 6 are adhered to thecore 2. Thereby, a building panel 1 having the core 2, the surface layer12 and the balancing layer 6 is formed, which is shown in FIG. 5 e.

After pressing, the balancing layer 6 balances the tension created bythe surface layer 12 such that the building panel 1 remains essentiallyflat after pressing and cooling (“pressing balancing” and “coolingbalancing”). By essentially flat is meant a cupping less than 2 mm/m. Asmall convex backward bending being less than 2 mm/m is preferred. Thethermosetting binder of the balancing layer 6 balances forces created bythe thermosetting binder of the surface layer 12. By using the same typeof binder in the balancing layer 6 as in the surface layer 12, theforces created by the binder in the surface layer 12 are matched andcounteracted by the forces created by the binder in the balancing layer6 on the opposite surface of the core.

In addition to the thermosetting binder, the balancing layer 6 maycomprise various additives. The balancing layer 6 is in this embodimentpaper free, i.e. comprises no paper. In a preferred embodiment, thebalancing layer 6 comprises only the thermosetting binder and optionaladditives. The balancing layer may consist of, or essentially consistof, the thermosetting binder. It is also contemplated that fillerparticles may be applied to the balancing layer 6, preferably in anamount lower than 50% by weight of the balancing layer 6, morepreferably in an amount lower than 20% by weight of the balancing layer6. Filler particles may include wood fibres, sand, mineral particles,aluminium oxide, etc. The filler particles may be scattered onto thelayer of the thermosetting binder such that the balancing layercomprises the thermosetting binder and filler particles. Alternatively,the filler particles may be mixed with the thermosetting binder inliquid form. If a large amount of thermosetting binder is required tobalance the surface layer 12, filler particles may be included in thebalancing layer 6 to obtain necessary tension of the balancing layer 6.

The methods result in a building panel 1 comprising a core 2, a surfacelayer 12 and a balancing layer 6 as shown in FIG. 1 e, FIG. 2 e and FIG.4 e. The core 2 is preferably a wood based board, preferably awood-based board such as MDF or HDF. The core 2 may also be an OSB orplywood. The balancing layer 6 is arranged on the first surface 3 of thecore 2. The surface layer 12 is arranged on the second surface 4 of thecore 2, opposite the first surface 3 of the core 2. The surface layer 12comprises preferably a décor paper 13. The décor paper 13 may bearranged directly on the second surface 4 of the core 2. The décor paper13 is preferably impregnated with resin, such as impregnated withmelamine formaldehyde resin. An overlay paper 14, preferably impregnatedwith a resin such as melamine formaldehyde resin and comprising wearresistant particles, may be arranged on the décor paper 14. In theembodiment described with reference to FIGS. 2 e and 4 e, the balancinglayer 6 may comprise at least 80 wt % of a thermosetting binder,preferably at least 90 wt % of a thermosetting binder. The thermosettingbinder of the balancing layer 6 is preferably the same as thethermosetting binder of the surface layer 12, for example, melamineformaldehyde resin.

In alternative embodiments, the surface layer 12 may be a wood fibrebased powder comprising wood fibres, a thermosetting resin and wearresistant particles. The surface layer 12 may also be a layer of athermosetting binder such as melamine formaldehyde or urea formaldehyde.The surface layer 12 may consist essentially of the thermosetting binderbut may also comprise decorative elements such as pigments and wearresistant particles such as aluminium oxide. The surface layer 12comprises in this embodiment no paper. It is contemplated that there arenumerous modifications of the embodiments described herein, which arestill within the scope of the invention as defined by the appendedclaims.

The balancing layer is referred to as a layer. However, during pressing,the thermosetting binder may in some embodiments at least partlyimpregnate the core such that the layer becomes less distinct. Thereby,the balancing layer may be at least partly integrated into the core.After pressing, the balancing layer may at least partly form part of thecore.

It is also contemplated that pre-pressing of the balancing layer may beperformed prior to curing the balancing layer and the surface layer.During pre-pressing step, the balancing layer remains in B-stage, or isat least not completely in C-stage. Cooling of the balancing layer maybe performed after the pre-pressing step.

Furthermore, it is also contemplated that an overlay paper such as aresin impregnated overlay paper may be included in the surface layer andthat the balancing layer balances both the décor layer and the overlaypaper.

Embodiments may also be defined as:

A method of producing a building comprises providing a core, applying athermosetting binder on a first surface of the core for forming abalancing layer, wherein the balancing layer comprises at least 80% byweight of a thermosetting binder, preferably at least 90% by weight of athermosetting binder, applying a surface layer on a second surface ofthe core, wherein the surface layer comprises a thermosetting binder,and curing the surface layer and the balancing layer by applying heatand pressure.

The balancing layer is adapted to counteract shrinking forces formed bythe surface layer during pressing and cooling such that the buildingpanel remains essentially flat in room temperature after pressing andcooling.

The amount of the thermosetting binder applied on the first surface ischosen such that the balancing layer balances shrinking forces formed bythe surface layer during pressing and cooling.

Also the chemical composition of the balancing layer, including, forexample, additives applied to the thermosetting binder, affectsproperties such as tension of the balancing layer.

An advantage of the method is that the balancing layer that consistsessentially of a thermosetting resin, counteracts and balances thetension created by the surface layer during pressing (“pressingbalancing”). The balancing layer keeps the building panel essentiallyflat after pressing. Thereafter, the balancing layer counteracts andbalances temperature shrinking of the surface layer when the panel iscooled from the present temperature to room temperature (“coolingbalancing”) such that the building panel remains essentially flat.Finally, the balancing layer counteracts and balances cupping of theedges of the floor panel due to shrinkage and expansion caused bychanges in temperature and humidity of the indoor climate (“climatebalancing”).

By applying essentially only a thermosetting binder as a balancinglayer, the cost for the balancing layer is reduced, for example,compared to using an impregnated paper as a balancing layer.Furthermore, the overall production of the building panel is simplifiedby removing the step of impregnating a paper for forming a balancingpaper layer. The production process is simplified by applying thethermosetting binder directly on the core for forming a balancing layer.

The tension of the balancing layer counteracting shrinking forces formedby the surface layer during pressing and cooling may be varied by theamount of thermosetting resin applied, additives applied, for example,additives changing the reactivity of the thermosetting binder, waterapplied, and the type of thermosetting binder. For example, a loweramount of thermosetting binder may be compensated by adding an additive,applying water, or choosing a thermosetting binder creating largertension forces such as melamine formaldehyde resin.

The balancing layer is referred to as a layer. However, during pressing,the thermosetting binder may at least partly impregnate the core suchthat the layer becomes less distinct. Thereby, the balancing layer maybe at least partly integrated into the core. After pressing, thebalancing layer may at least partly form part of the core.

The balancing layer may consist essentially of a thermosetting binder.Additives such as, for example, wetting agents, release agents,catalysts, etc., may be included in the balancing layer that consistsessentially of the thermosetting binder. Catalyst may influence howlarge tension forces that may be formed by the balancing layer adaptedto counteract shrinking forces of the surface layer.

The pressing temperature may exceed 140° C., for example, 140-210° C.

The amount of the thermosetting binder in the balancing layer may exceed50 g/m2. The amount of the thermosetting binder in the balancing layer 6may about 75% of the amount of the thermosetting binder in the surfacelayer.

The thermosetting binder is preferably an amino resin such as ureaformaldehyde or melamine formaldehyde.

The thermosetting binder may include pigments. The thermosetting bindermay include wear resistant particles such as aluminium oxide.

In one embodiment, the balancing layer consists of, or consistsessentially of the thermosetting binder. In this embodiment, thebalancing layer comprises no paper and no wood fibres. However, abalancing layer consisting essentially of the thermosetting binder mayinclude additives.

The thermosetting binder may be of the same type in the surface layerand in the balancing layer. By the expression same type is meant binderbelong to the same group of resin, such as melamine formaldehyde, ureaformaldehyde, etc. By using the same type of binder in both the surfacelayer and the balancing layer, the balancing layer matches the behaviorof the surface layer The balancing layer balances shrinkage and/orexpansion of the balancing layer by counteracting the movements of thesurface layer in a similar way when using the same type of thermosettingbinder.

The thermosetting binder of the surface layer may be melamineformaldehyde resin. Direct Pressure Laminate (DPL) and High PressureLaminate (HPL) are conventionally impregnated with melamine formaldehyderesin.

The thermosetting binder of the surface layer and/or the balancing layermay also be urea melamine formaldehyde or a combination of urea/melamineformaldehyde.

The thermosetting binder of the balancing layer may be melamineformaldehyde resin. A balancing layer comprising melamine formaldehyderesin creates greater tension compared to, for example, ureaformaldehyde. Therefore, a balancing layer comprising melamineformaldehyde can counteract/balance greater tension formed by thesurface layer.

The surface layer may be arranged directly on the core. The balancinglayer can balance a surface layer arranged directly on the core. Thesurface layer may comprise a décor paper, preferably a resin impregnateddécor paper, more preferably a melamine formaldehyde impregnated décorpaper. The décor paper may be arranged directly on the core. It has beenshown that the balancing layer consisting essentially of a thermosettingbinder can be used to balance a surface layer of décor paper. Thus, abalancing layer consisting essentially of a thermosetting binder can beused to balance a DPL.

The surface layer may comprise a layer of a thermosetting binder,preferably melamine formaldehyde resin, and at least one pigment. Thesurface may in this embodiment consist essentially of the thermosettingbinder with optional additives, such as pigments, wear resistantparticles, etc.

The amount of the thermosetting binder in the balancing layer mayessentially correspond to the amount of the thermosetting binder in thesurface layer. Thereby, balancing of the surface layer is furtherimproved.

The amount of the thermosetting binder in the balancing layer may belower than the amount of the thermosetting binder in the surface layer.The amount of the thermosetting binder in the balancing layer may, forexample, be about 80% of the amount of the thermosetting binder in thesurface layer.

Thermosetting binder of the balancing layer may be applied in liquidform. Thereby, the balancing layer may be applied as a coating to thecore. The balancing layer may be applied as one or several layers of thethermosetting binder. By using a liquid thermosetting binder, no extraoff-line drying of the binder is necessary, for example, spray drying ordrying of an impregnated paper.

The method may further comprise drying the thermosetting binder.Preferably, the balancing layer remains essentially in B-stage. Thebalancing layer may be applied as several layers of a thermosettingbinder, wherein each layer preferably is dried before application of asubsequent layer.

The step of applying the thermosetting binder forming the balancinglayer may comprise applying, preferably by scattering, the thermosettingbinder in powder form. The thermosetting binder may be a dry powder. Byapplying the thermosetting binder in powder form, the balancing layermay be applied in a single step, preferably as a single layer. Thethermosetting binder may be spray dried.

The method may further comprise applying water or steam, on the firstsurface of the core prior to applying to the thermosetting binder.Alternatively, the method may further comprise applying preferablywater, or steam, on the thermosetting binder. The aqueous solution orsteam is used to stabilize the balancing layer such that the core withthe balancing layer can be handled in the production line, for example,be turned around. The aqueous solution or steam makes the thermosettingbinder tacky and thereby the thermosetting binder in powder form stickstogether. The aqueous solution may comprise release and wetting agentsand catalysts. The aqueous solution may include pigments. Thethermosetting binder remains essentially in B-stage.

The moisture content of the balancing layer may be higher than themoisture content of the surface layer as measured prior to pressing. Themoisture content may be 3-30%, such as 8-20%, of the total weight of thebalancing layer. The moisture content of the balancing layer may exceed10%, preferably 20%, more preferably 30%, of the total weight of thebalancing layer. Water or steam may be applied to the thermosettingbinder or to the first surface of the core prior to applying thethermosetting binder in order to adjust the moisture content of thebalancing layer to be formed.

Pressing test show that a suitable moisture content in a balancing layercomprising a thermosetting binder such as melamine formaldehyde mayincrease the shrinking forces during curing and cooling and that thismay be used to reduce the content of the thermosetting binder in thebalancing layer. It is preferred that the moisture content in thethermosetting binder of the balancing layer prior to pressing is higherthan the general moisture content of about 6% that is the case whenconventional impregnated paper is used as a balancing layer. Themoisture content in liquid or spray dried thermosetting binder such asmelamine should preferably exceed the moisture content of the surfacelayer. The moisture content should preferably exceed 10% of the totalweight of the balancing layer. In some application it may be anadvantage to use even higher moisture contents, for example, exceeding20%.

According to another embodiment, a building panel is provided. Thebuilding panel comprises a core having a first and second surface, abalancing layer arranged on the first surface of the core, wherein thebalancing layer comprises at least 80% by weight of a thermosettingbinder, preferably at least 90% by weight of a thermosetting binder, asurface layer arranged on the second surface of the core, wherein thesurface layer comprises a thermosetting binder.

The balancing layer is adapted to counteract shrinking forces formed bythe surface layer during pressing and cooling such that the buildingpanel remains essentially flat in room temperature after pressing.

The balancing layer is referred to as a layer. However, during pressing,the thermosetting binder may at least partly impregnate the core suchthat the layer becomes less distinct. Thereby, the balancing layer maybe at least partly integrated into the core. After pressing, thebalancing layer may at least partly form part of the core.

The thermosetting binder may be of the same type in the surface layerand in the balancing layer.

The thermosetting binder of the surface layer may be melamineformaldehyde resin.

The thermosetting binder of the balancing layer may be melamineformaldehyde resin.

The surface layer may be arranged directly on the core.

The surface layer may comprise a décor paper, preferably a resinimpregnated décor paper, more preferably a melamine formaldehydeimpregnated décor paper.

The amount of the thermosetting binder in the balancing layer mayessentially correspond to the amount of the thermosetting binder in thesurface layer. The amount of the thermosetting binder in the balancinglayer may be lower than to the amount of the thermosetting binder in thesurface layer. The amount of the thermosetting binder in the balancinglayer may, for example, be about 80% of the amount of the thermosettingbinder in the surface layer.

The moisture content of the balancing layer may be higher than themoisture content of the surface layer as measured prior to pressing. Themoisture content may be 6-30%, such as 8-20%, of the total weight of thebalancing layer prior to curing.

The moisture content of the balancing layer may exceed 10%, preferably20%, more preferably 30%, of the total weight of the balancing layerprior to curing.

The surface layer may comprise a layer of a thermosetting binder and atleast one pigment. The surface layer may in one embodiment consist of,or consist essentially of a thermosetting binder and optional additives.

According to another embodiment, a semi-finished product is provided.The semi-finished product comprises a core having a first surface, and abalancing layer arranged on the first surface of the core, the balancinglayer comprising at least 80% by weight of a thermosetting binder,preferably at least 90% of a thermosetting binder.

The semi-finished product is already provided with a balancing layer. Asurface layer can in a separate process be applied to the semi-finishedproduct to form a building panel. The semi-finished product may, forexample, be stored and transported.

The thermosetting binder may be essentially B-stage. For example, thebalancing layer may only be dried without curing, or if thethermosetting binder is in powder form, liquid or steam may be appliedin order to get the binder tacky and stick the powder together andattach to the core.

The thermosetting binder may be melamine formaldehyde resin.

According to another embodiment, a method of producing a building panelis provided. The method comprises: providing a core, applying athermosetting binder in a liquid form on a first surface of the core forforming a balancing layer, applying a surface layer on a second surfaceof the core, wherein the surface layer comprises a thermosetting binder,and curing the surface layer and the balancing layer by applying heatand pressure.

The balancing layer is adapted to counteract shrinking forces formed bythe surface layer during pressing and cooling such that the buildingpanel remains essentially flat in room temperature after pressing.

Furthermore, by applying the thermosetting binder as a liquid, no extraoff-line drying of the binder is necessary, for example, spray drying ordrying of an impregnated paper.

The balancing layer is referred to as a layer. However, during pressing,the thermosetting binder may at least partly impregnate the core suchthat the layer becomes less distinct. Thereby, the balancing layer maybe at least partly integrated into the core. After pressing, thebalancing layer may at least partly form part of the core.

The balancing layer may be paper free.

The method may further comprise drying the thermosetting binder prior toapplying heat and pressure.

The method may further comprise applying filler particles on thethermosetting binder. The filler particles may be wood fibres, sand,mineral particles, aluminium oxide, etc. Alternatively, the fillerparticles may be mixed with the thermosetting binder.

The balancing layer may comprise at least 80% by weight of thethermosetting binder, preferably at least 90% by weight of thethermosetting binder.

The building panel may be pressed at a temperature of at least 120° C.

The thermosetting binder may be present with the balancing layer with anamount of at least 50 g/m2.

The thermosetting binder may be of the same type in the surface layerand in the balancing layer.

The thermosetting binder of the surface layer and/or the balancing layeris melamine formaldehyde resin. The thermosetting binder of the surfacebinder and/or the balancing layer may also be urea melamine formaldehydeor a combination of urea/melamine formaldehyde.

The surface layer may be arranged directly on the core.

The surface layer may comprise a décor paper, preferably a resinimpregnated décor paper, more preferably a melamine formaldehydeimpregnated décor paper.

The surface layer may comprise a layer of a thermosetting binder and atleast one pigment.

The amount of the thermosetting binder in the balancing layer mayessentially correspond to the amount of the thermosetting binder in thesurface layer.

The amount of the thermosetting binder in the balancing layer may belower than the amount of the thermosetting binder in the surface layer.

The moisture content of the balancing layer may be higher than themoisture content of the surface layer.

The moisture content may be 6-30%, such as 8-20%, of the total weight ofthe balancing layer prior to curing.

The moisture content of the balancing layer may exceed 10%, preferably20%, more preferably 30%, of the total weight of the balancing layerprior to curing.

EXAMPLES Example 1 Dry Melamine Formaldehyde Resin

A water solution was sprayed on one surface of a standard HDF board,Sonae 9.7 mm. Then 100 g/m2 melamine formaldehyde powder resin—773 fromBASF—was applied with a scattering device on the wet HDF board.

The HDF board was then turned upside down and a surface layer containing100 g/m2 melamine formaldehyde resin was applied on the other side ofthe board.

The HDF board containing a clean dry melamine formaldehyde resin on oneside for forming a balancing layer and on the other side a surface layercontaining 100 g/m2 melamine, then was put into a press where heat andpressure was applied.

Pressing conditions:

Temperature: 160° C. on the upper and under heating plates.

Pressing time: 20 sec

Pressure: 40 bar

During the pressing operation the melamine formaldehyde resins cured anda laminate board was obtained.

In order to investigate the cupping behavior of this product we placedthe above flooring laminate into different climates. After 3 days in 50%RH climate the cupping was +0.83 mm (convex cupping). After 2 days in25% RH climate the cupping was −0.07 mm (slightly concave).

Example 2 Wet Melamine Formaldehyde Resin

300 g/m2 wet melamine formaldehyde resin with a solid content of 50% wasapplied with a roller device one surface of a standard HDF board, Sonae9.7 mm, and then dried in an oven so the surface was dry which gives thepossibility to turn the HDF board upside down. A surface layercontaining 300 g/m2 melamine formaldehyde resin was applied on the otherside of the board.

The HDF board containing a wet melamine resin on one side for forming abalancing layer and on the other side a surface layer containing 300g/m2 melamine resin was then put into a press where heat and pressurewas applied.

Pressing conditions:

Temperature: 170° C. on the upper and under heating plates.

Pressing time: 30 sec

Pressure: 40 bar

During the pressing operation the melamine formaldehyde resins cured anda laminate board was obtained.

In order to investigate the cupping behavior of this product we placedthe above flooring laminate into different climates. After 3 days in 50%RH climate the cupping was −0.7 mm (concave cupping). After 16 days in25% RH climate the cupping was −0.47 mm (concave cupping).

Example 3 Water

In order to investigate the influence of water, water was added to a topsurface layer. A top surface layer containing 100 g/m2 melamineformaldehyde resin was applied on a surface of a standard HDF board,Sonae 9.7 mm. On top of this a layer 15 g/m2 of water was applied with aspraying device. On the opposite surface of the board a surface layercontaining 100 g/m2 melamine was applied.

The board was then put into a press where heat and pressure was applied.

Pressing conditions:

Temperature: 170° C. on the upper and under heating plates.

Pressing time: 30 sec

Pressure: 40 bar

During the pressing operation the melamine formaldehyde resins cured anda laminate board was obtained.

In order to investigate the cupping behavior of this product we placedthe above board into different climates. After 3 days in 50% RH climatethe cupping was −1.33 mm (concave cupping). After 3 days in 25% RHClimate the cupping was −2.11 mm (concave cupping).

The above trial was repeated with higher water amounts added on a topsurface of a HDF board, in increased steps:

30 g/m2 of water applied was applied on the HDF board giving thefollowing cupping:

After 3 days in 50% RH climate the cupping was −1.64 mm (concavecupping). After 3 days in 25% RH Climate the cupping was −2.51 mm(concave cupping).

45 g/m2 of water applied giving the following cupping:

After 3 days in 50% RH climate the cupping was −3.17 mm (concavecupping). After 3 days in 25% RH Climate the cupping was −4.30 mm(concave cupping). 60 g/m2 giving the following cupping:

After 3 days in 50% RH climate the cupping was −3.24 mm (concavecupping). After 3 days in 25% RH Climate the cupping was −4.55 mm(concave cupping).

As can be seen in the above examples, the more water added, the moreconcave cupping is created on the side where water is added, i.e. inthis example on the top surface.

Example 4 Water

Five different examples were tested. The examples had the followingbuild-up:

A(Reference) B C D E Overlay MF impregnated MF impregnated MFimpregnated MF impregnated MF impregnated overlay paper overlay paperoverlay paper overlay paper overlay paper Décor layer MF impregnated MFimpregnated MF impregnated MF impregnated MF impregnated décor paperdécor paper décor paper décor paper décor paper Core HDF HDF HDF HDF HDFWater applied 0 15  0 15 15 on core (g/m2) Water applied 0  0 15 15 15on balancing layer (g/m2) Balancing layer MF impregnated MF impregnatedMF impregnated MF impregnated MF impregnated backing paper backing paperbacking paper backing paper overlay paper MF = melamine formaldehyderesin.The water applied also contained about 1 wt % of a catalyst, about 3-6wt % of a release agent, and about 2.5 wt % of a wetting agent.The different layers had the following composition:

Layer Overlay Décor Backing Raw paper (g/m2) 73/74 61 Treated paper(g/m2) 104  143/142 173 MF resin (g/m2)  67* 69/69 112 Resin content (%)75 49/48 65 *Estimated raw paper 22 g/m2 plus 15 g/m2 of Al2O3 as wearresistant particles.The core was a 7.6 mm HDF board.The examples were pressed with the following press conditions:

Temperature: Oil temperature on upper heating plate 190° C. and 208° C.on the lower heating plate.

Pressing time: 12 sec

Pressure: 35 bar

After pressing, the impact of the water applied can be studied bycomparing the shape of the panels. The counteracting forces formed bythe melamine formaldehyde resin of the balancing layer, adapted tocounteract and balance the tension formed by the melamine formaldehyderesin of the oppositely arranged overlay and décor layers during curing,increased in the following order:

Example D was more convex in shape than example C, which was equal toexample B. Example B was more convex in shape than example A.Consequently, the higher amount of water applied, the largercounteracting forces were formed, thus resulting in a more convex shapeof the panel after pressing and curing. Example E was equal to example Ain its convex shape.

Example E shows that by spraying the balancing layer and the surface onthe core on which the balancing layer is arranged with water, an overlaypaper can replace a standard backing paper resulting in an equal convexshape. In example 5, the amount of melamine formaldehyde resin has beenreduced from 112 g/m2 to 67 g/m2, corresponding to a reduction of 40%,by replacing the standard backing paper with an overlay paper. The paperweight has been reduced from 61 g/m2 to approximately 22 g/m2. However,the resulting counteracting forces essentially equal a standard backingpaper.

Embodiments

1. A method of producing a building panel (1), comprising:

providing a core (2),

applying a balancing layer (6) on a first surface (3) of the core (2),wherein the balancing layer (6) has a first moisture content andcomprises a B-stage thermosetting binder,

applying a surface layer (12) on a second surface (4) of the core (2),wherein the surface layer (12) has a second moisture content andcomprises a B-stage thermosetting binder,

adjusting the first moisture content of the balancing layer (6) suchthat the first moisture content of the balancing layer (6) is higherthan the second moisture content of the surface layer (12) prior tocuring by applying heat and pressure, and curing the thermosettingbinder of the surface layer (12) and the balancing layer (6) to C-stageby applying heat and pressure.

2. A method according to embodiment 1, wherein the step of adjusting thefirst moisture content of the balancing layer comprises applying wateror steam to the first surface (3) of the core (2) prior to applying thebalancing layer (6).

3. A method according to embodiment 1 or 2, wherein the step ofadjusting the first moisture content of the balancing layer comprisesapplying water or steam to the balancing layer (6).

4. A method according to any one of embodiment 1-3, wherein the firstmoisture content of the balancing layer (6) is 6-30%, preferably 8-20%,of the total weight of the balancing layer (6) prior to curing byapplying heat and pressure.

5. A method according to any one of embodiment 1-4, wherein thebalancing layer (6) comprises a resin impregnated sheet, preferably aresin impregnated paper.

6. A method according to any one of embodiment 1-4, wherein the step ofapplying the balancing layer (6) comprises applying the thermosettingbinder in powder form.

7. A method according to any one of embodiment 1-6, wherein the amountof the thermosetting binder in the balancing layer (6) is lower than theamount of the thermosetting binder in the surface layer (12).

The invention claimed is:
 1. A method of producing a building panel,comprising: providing a core, applying a balancing layer on a firstsurface of the core, wherein the balancing layer has a first moisturecontent and comprises a sheet impregnated with a thermosetting binder,applying a surface layer on a second surface of the core, wherein thesurface layer has a second moisture content and comprises athermosetting binder, adjusting the first moisture content of thebalancing layer such that the first moisture content of the balancinglayer is higher than the second moisture content of the surface layerprior to curing by applying heat and pressure, and curing the surfacelayer and the balancing layer by applying heat and pressure.
 2. A methodaccording to claim 1, wherein the step of adjusting the first moisturecontent of the balancing layer comprises applying water or steam to thefirst surface of the core prior to applying the balancing layer.
 3. Amethod according to claim 1, wherein the step of adjusting the firstmoisture content of the balancing layer comprises applying water orsteam to the balancing layer.
 4. A method according to claim 1, whereinthe first moisture content of the balancing layer exceeds 10% of thetotal weight of the balancing layer prior to curing.
 5. A methodaccording to claim 1, wherein the thermosetting binder is of the sametype in the surface layer and in the balancing layer.
 6. A methodaccording to claim 1, wherein the thermosetting binder of the surfacelayer and/or the balancing layer is melamine formaldehyde resin.
 7. Amethod according to claim 1, wherein the sheet is a paper sheet.
 8. Amethod according to claim 1, wherein the surface layer comprises a décorpaper.
 9. A method according to claim 1, wherein the surface layercomprises a layer comprising a thermosetting binder and at least onepigment.
 10. A method according to claim 1, wherein the amount of thethermosetting binder in the balancing layer is lower than the amount ofthe thermosetting binder in the surface layer.
 11. A method according toclaim 2, wherein the step of adjusting the first moisture content of thebalancing layer comprises applying water or steam to the balancinglayer.
 12. A method according to claim 1, wherein the first moisturecontent of the balancing layer exceeds 20% of the total weight of thebalancing layer prior to curing.
 13. A method according to claim 1,wherein the first moisture content of the balancing layer exceeds 30% ofthe total weight of the balancing layer prior to curing.
 14. A methodaccording to claim 1, wherein the surface layer comprises a resinimpregnated décor paper.
 15. A method according to claim 1, wherein thesurface layer comprises a melamine formaldehyde impregnated décor paper.